When trimethylolpropane (hereinafter may be referred to as “TMP”) is industrially produced through aldol condensation and crossed Cannizzaro reaction between n-butyraldehyde (hereinafter may be referred to as “NBD”) and formaldehyde in the presence of a base catalyst, ditrimethylolpropane (hereinafter may be referred to as “di-TMP”) is produced as a by-product, and di-TMP is recovered from a high-boiling-point mixture (Patent Document 1).
In a method for recovering di-TMP from such a high-boiling-point mixture, a reaction mixture of NBD and formaldehyde is subjected to solvent extraction after concentration or without concentration, to thereby produce a TMP extract (crude TMP) containing substantially no sodium formate, and the crude TMP is purified through distillation under high vacuum, to thereby produce a distillation residue containing 1 to 20% TMP and 20 to 50% di-TMP.
Proposed methods for recovering di-TMP from such a crude TMP distillation residue include a method of crystallization of di-TMP from ethyl acetate (Patent Document 2); a method of crystallization of di-TMP from water (solvent) in the presence of sodium formate (Patent Document 3); a method of crystallization of di-TMP from 1,4-dioxane (solvent) (Patent Document 4); and a method of crystallization of di-TMP from acetone (solvent) (Patent Document 5).
Such a method for recovering di-TMP from a distillation residue of TMP (i.e., main target product) is current employed as a general procedure for producing di-TMP. In such a di-TMP production method, only di-TMP must be separated and recovered from the TMP distillation residue, which contains formaldehyde and NBD (i.e., raw materials for production of TMP), modified products generated during recovery (through distillation) of TMP, and by-products other than di-TMP (e.g., acetal formed between TMP and formaldehyde). However, a low di-TMP content of the distillation residue makes it difficult to recover di-TMP at an industrially satisfactory yield.
Particularly, the distillation residue obtained upon TMP production contains bistrimethylolpropane (hereinafter may be referred to as “bis-TMP”), which is a linear formal formed from two TMP molecules and formaldehyde, and bis-TMP and di-TMP are difficult to separate from each other. Therefore, effective production of di-TMP requires considerable reduction in amount of bis-TMP by-produced.
Attempts have been made to effectively produce di-TMP by increasing the amount of di-TMP by-produced upon TMP production, and there has been proposed a method in which reaction is carried out under certain specific conditions (Patent Documents 6 and 7), as well as a method in which a water-insoluble organic solvent is added to the reaction system (Patent Document 8).
In such proposed methods, di-TMP is produced at a yield of about 10 mol % on the basis of NBD. That is, the amount of di-TMP by-produced through such a method is only twice or thrice that of di-TMP by-produced through the aforementioned conventional TMP production methods in which no measures are taken to increase the amount of di-TMP by-produced. Thus, di-TMP is a by-product of TMP production, and the amount of di-TMP produced is necessarily limited by the amount of TMP produced. Therefore, the aforementioned TMP production methods cannot meet increased demand for di-TMP.
The method described in Patent Document 8 in which a water-insoluble organic solvent is added to the reaction system for extraction of di-TMP is industrially very disadvantageous in that, after extraction, both the aqueous layer and the organic layer contain di-TMP (i.e., a compound of interest), and thus the method requires a very intricate post treatment after reaction and also requires recovery (through distillation) of a used organic solvent.
Meanwhile, there have been disclosed di-TMP synthesis methods, including a method in which an ether bond is formed through dehydration-condensation of two TMP molecules (Patent Document 9), and a method for synthesizing di-TMP through reaction between 2-ethyl-2-propenal (hereinafter may be referred to as “ECR”) and TMP (Patent Document 10). Unlike the aforementioned methods for recovering di-TMP from a TMP distillation residue, such a di-TMP synthesis methods can meet increased demand for di-TMP, since the amount of di-TMP produced through the synthesis methods is not limited by the amount of TMP produced.
However, in the method in which an ether bond is formed through dehydration-condensation of two TMP molecules, since di-TMP is produced through reaction between TMP molecules, each having a structure including three alcoholic hydroxyl groups which can participate in reaction, by-production of an ether condensate of three or more TMP molecules, which would otherwise occurs inevitably, cannot be prevented. For suppression of such by-production, the percent reaction of dehydration-condensation between TMP molecules must be reduced, but accordingly high cost is required for recovery of unreacted TMP through distillation, resulting in an industrial disadvantage.
In order to solve such a problem, Patent Document 9 describes a method which uses, as a raw material, TMP in which some of three alcoholic hydroxyl groups have been esterified with a lower fatty acid in advance. However, this method cannot essentially solve the aforementioned problem of by-production of an ether condensate of three or more TMP molecules, due to the inability to selectively esterify only two alcoholic hydroxyl groups of one TMP molecule with a lower fatty acid. In addition, the method described in Patent Document 9 produces TMP in which one or two alcoholic hydroxyl groups have been esterified, and thus requires a new process for regenerating di-TMP through hydrolysis of di-TMP having esterified alcoholic hydroxyl groups, resulting in increased production cost and an industrial disadvantage.
In the method disclosed in Patent Document 10, even when TMP is used in a large excess amount with respect to ECR, the yield of di-TMP is less than 70% on the basis of ECR. This method is economically disadvantageous in that the method requires recovery of di-TMP, as well as recovery of excessively used TMP through distillation.
Patent Document 1: U.S. Pat. No. 3,097,245, specification
Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. S47-30611
Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. S49-133311
Patent Document 4: Japanese Patent Application Laid-Open (kokai) No. 2002-47231
Patent Document 5: Japanese Patent Application Laid-Open (kokai) No. 2005-23067
Patent Document 6: Japanese Patent Application Laid-Open (kokai) No. S57-139028
Patent Document 7: Japanese Patent Application Laid-Open (kokai) No. S57-142929
Patent Document 8: Japanese Patent Application Laid-Open (kokai) No. H08-157401
Patent Document 9: Japanese Kohyo Patent Publication No. H06-501470
Patent Document 10: Japanese Patent Application Laid-Open (kokai) No. H09-268150